Emission control apparatus which automatically regulates the fuel-air ratio of an internal combustion engine to the stoichiometric ratio under all conditions of operation is presently known.sup.1. Control is on the basis of an EMF developed by an oxygen concentration cell which provides a sharp voltage change at the stoichiometric air-fuel ratio. The cell includes a solid electrolyte having zero open porosity, and capable of conducting oxygen ions between an interior portion of the solid electrolyte which is in contact with ambient air and an exterior portion thereof which is in contact with exhaust gases from the internal combustion engine. An EMF is developed between suitable electrodes, one on the interior of the solid electrolyte and one on the exterior thereof; the magnitude of this EMF is monitored, and the air-fuel ratio is controlled to maintain the magnitude of the monitored EMF at the one which indicates the stoichiometric air-fuel ratio. FNT .sup.1 See H. Dueker, K. H. Friese and W. D. Haeker, Paper 750223 presented at S.A.E. Automotive Engineering Congress and Exposition, Detroit, 1975 and cited references, and U.S. Pat. Nos. 3,960,692 and 3,960,693.
Stabilized.sup.2 zirconia ceramics have been suggested for use as a solid electrolytes in oxygen sensors of the type in question. To be suitable for use as a solid electrolyte, a ceramic must be impervious because, otherwise, diffusion of gaseous oxygen through the ceramic will cause polarization of an associated electrode and, consequently, a reduced EMF. Several sintering aids have been suggested for use in stabilized zirconia ceramics to enable the production of impervious bodies at reasonable firing temperatures. Examples of such sintering aids.sup.3 include alumina, titania, silica, combinations of alumina and titania, combinations of alumina and silica in which alumina is not more than 50 percent of the weight of the sintering aid and a glass containing CaO, MgO, Al.sub.2 O.sub.3 and SiO.sub.2 in certain proportions. FNT .sup.2 A zirconium oxide ceramic, like a hafnium oxide ceramic, if not stabilized, undergoes a destructive tetragonal to monoclinic inversion. Yttrium oxide, calcium oxide, magnesium oxide and rare earth oxides have been suggested as stabilizers for zirconia ceramics. FNT .sup.3 See for example, U.S. Pat. Nos. 3,565,645, 3,607,323 and 3,843,400, as well as British Patent No. 1,385,464
It has been found.sup.4 that yttria, calcium oxide and magnesium oxide can be used to transfer hafnium oxide into a cubic crystal structure which is not subject to destructive transformation, and that strong gas-tight tubes can be prepared by a slip casting technique.sup.5 from an aqueous slurry containing HfO.sub.2, CaO, ammonium alginate, formaldehyde and bentonite. FNT .sup.4 See, for example, D. W. Stacy and D. R. Wilder, "The Yttria-Hafnia System", Journal of the American Ceramic Society, Vol. 58, No. b 7-8, pp. 285-288 (1975) and J. D. Schieltz, J. W. Patterson and D. R. Wilder, "Electrolytic Behavior of Yttria-Stabilized Hafnia", J. Electrochem, Soc.: Electrochemical Science, Vol. 118, No. 8, pp. 1257-1261 (1971). FNT .sup.5 U.S. Pat. No. 3,287,143